In the past, there have been proposed various device management systems for managing (at least either of monitoring or controlling) a plurality of devices, and the device management systems construct a lower-level communication network with respect to each of management groups which are grouped in terms of kinds of devices and arranged places. The plurality of devices are arranged, and are divided into a plurality of management groups. The lower-level communication network is managed by an upper-level communication network. In order to construct such hierarchical communication network, a technology to manage a slave unit by a master unit has been used. In the technology, the slave unit for managing a device (the slave unit integrated with the device is also included) is arranged in the lower-level communication network. The master unit treats the slave unit as a terminal, and is arranged in the upper-level communication network. BACnet protocol (A Data Communication Protocol for Building Automation and Control Networks) is used as a communication protocol between the master and slave units (for example, see Japanese Patent Application Laid-Open No. 2007-96539).
However, if the master unit malfunctions, such old device management system can not maintain a management of a slave unit which had been managed by the master unit until then. As a result, the old device management system has a problem that the slave unit (which had been managed by the malfunctioning master unit) can not maintain managements of devices had been managed until then.